Is your younger brother 18 or over? If so, he needs to fill his IMM 5406E
Why does you sister need to fill one? Why do you need to fill one?
Are you two part of the application? (dependent of your mom)
Did you see IRCC's definition of family member?
https://www.canada.ca/en/services/immigration-citizenship/helpcentre/glossary.html#f
Family membersAn applicant’s closest relatives, in the context of an application to IRCC. It is defined as a spouse or common-law partner, dependent children, and their dependent children.
And below (search for family member)
https://www.canada.ca/en/immigratio...efinitions-related-permanent-residence.html#f
Once you are not a dependent, you are not see as a family member under IRCC.